1. Field of the Invention
The present invention relates to a PCM (Pulse Code Modulation)-based communications system and, more particularly, to a system and a method which enable data to be transmitted and received using PCM coding.
2. Description of the Related Art
In recent years, as IP networks advance, the VoIP (Voice over IP) technologies are becoming important factors in network communications. In parallel, there is a growing trend among VoIP telephony service providers to guarantee communication quality by employing the PCM-based recommendation G.711 as a voice encoding method and securing a bandwidth for UDP (User Datagram Protocol) packets. Meanwhile, as mobile telephones, or those with digital cameras, become widespread, the uses of the service of transmitting instant messages, still images and the like are dramatically increasing in number.
Therefore, there is a need for a simple data-transmission technology that places no burden on a network. In ISDN (Integrated Services Digital Network), a B channel enables both a conversation service and a data communication service to be used. However, it has a problem that a burden on a network is increased because there is a need to setting a service mode per call at the network side.
As a method for switching between communication modes (voice communication mode and data communication mode) on the terminal sides without network-side control, for example, Japanese Patent Unexamined Application Publication No. S63-69364 discloses a communication mode switching system. According to this prior art, switching control is performed as follows. When switching to the data communication mode during the voice communication mode, the switching is done by transmitting a signal that can pass through a voice band, such as a multifrequency signal or a push-button signal. When switching to the voice communication mode during the data communication mode, the switching is done by transmitting a specific signal code.
Moreover, Japanese Patent Unexamined Application Publication No. H7-58804 discloses a voice-data composite communication system in which a start flag FS and an end flag FE, each having six or more consecutive “1”s, are added before and after a data signal, respectively, thereby making it possible for the receiving side to distinguish between voice and data.
However, in any of the above-mentioned prior arts, no consideration is given to PCM-based voice communications.
Moreover, in the communication mode switching system described in Japanese Patent Unexamined Application Publication No. S63-69364, special control signals (such as the push-button signal and the specific signal code), apart from communicated information, are transmitted to the other end of the line through a network, resulting in increased traffic on the network.
In the voice data composite communications system described in Japanese Patent Unexamined Application Publication No. H7-58804, it is necessary to add the start flag FS and the end flag FE, each having six or more consecutive “1”s as not occurring in voice signals and data signals. In other words, a string of consecutive bits at an arbitrary value, which does not exist in a voice signal and a data signal, is used as a flag. Accordingly, it is necessary to provide the receiving side with a means for detecting such special start and end flags, resulting in complicated circuit structure.